The present invention relates to the method for the manufacture of a front fork blank of the single unit type for use in bicycles, and more particularly to the oil hydraulic bulge-forming process by means of a sectional forming die of top and bottom molds in which a tubular ferrous blank is used as a starting material.
Generally speaking, the known front fork has been heretofore manufactured by the assembling process wherein a fork crown with a hole into which a fork stem is firmly fixed, and two legs or blades are secured to the underside holes of the fork crown, respectively. The fork crown consists of four parts, stem, crown, and two blades.
In this age of resources saving, manpower saving, and energy saving, in the bicycle parts industry, the introduction of a process for the manufacture of a bicycle part which aims at saving the above factors should be preferred as the first and foremost priority. Of the four parts comprising the front fork, the crown has heretofore been manufactured by the two processes as follows: (1) Process for making the crown by metal casting in a mold which corresponds to the contour of the crown; and (2) Making the crown by blanking a steel sheet to the required size, subjecting the blanked sheet to several drawing steps by means of a press machine, and finally welding it to form a desired crown article.
Therefore it is seen that the mass production of the front fork on an inexpensive cost basis will not be expected. However, in accordance with the oil hydraulic bulge-forming process disclosed in U.S. Pat. No. 4,051,704 (patented Oct. 4, 1977) invented by the colleague, Senkichiro Kimura of the same factory to which I also belong, the front fork blank of the single unit type of this invention can be manufactured from a straight tubular ferrous blank by bending it into a two-prong fork, and subjecting the thus formed fork to the oil hydrualic bulge-forming process.
The oil hydrualic bulge-forming process which comprises subjecting a metal tubular stock set in a die to a high pressure working oil in order to make a protuberant component expand, namely, bulge on the metal tubular stock is now known in industry, but at first it was considered incredible even by professors of metallurgy of the university.